Advances in Quadrature Amplitude Modulation (QAM), Orthogonal Frequency-Division Multiplexing (OFDM), channel bonding, and Multiple-Input Multiple-Output (MIMO) are pushing wireless PHY data rates toward a new milestone. The fast increasing data rates make MAC overhead rise from a minor factor to a major factor that affects the efficiency of contention-based wireless communication. Taking Wi-Fi as an example, the transmission time for a 1500 byte packet has been reduced from 12 ms at 1 Mbps to 20 μs at 600 Mbps in a dozen years. The transmission time will be further shortened with higher Gbps PHY data rates supported in ongoing standardization of 802.11ac and 802.11ad. The average channel access overhead, however, stays at the same order of magnitude as before. In 802.11n, the average channel access overhead is 101.5 μs, which is about 5 times of the data transmission time for a 1500 byte packet at 600 Mbps. Although frame aggregation can reduce the relative impact of channel access overhead, it does not work well for non-bulk data flows or traffic that has stringent deadline requirements (e.g., short HTTP flows and VoIP packets).
In contention-based wireless networks, Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) is widely adopted to reduce collisions between nodes. Each node defers its transmission for a random number of slots to avoid collision and each slot must be long enough for accurate detection of busy channel. The random backoff is necessary because collision detection was thought to be impossible in wireless communication. Without collision detection, nodes cannot start transmission immediately when the channel is detected to be idle; otherwise, none of them can succeed transmission. The reason that collision detection was impractical is because even if a node could listen on the channel while transmitting, the strong self-interference would mask all other signals on the air. However, recent advances in self-interference cancellation have made full duplex wireless communication possible. This allows transmitters to parallelize transmission and detection. By designing different preambles before data transmission, different contention resolution schemes have been proposed.
WiFi-Nano relies on cross-correlation to compute the start time of a preamble transmission. It still needs random backoff to spread out nodes' transmissions but the random backoff time is shortened by reducing the backoff slot duration to 800 ns. Although the adoption of tiny time slots in random backoff helps to reduce channel access overhead, the random backoff can no longer desynchronize hidden terminals. As a result, hidden terminals may keep colliding with each other. In addition, a weak signal can be discovered only after all stronger transmissions are aborted but transmissions initiated in the same time slot are aborted probabilistically. Therefore, the time used to resolve a collision is uncertain and it is possible that all nodes abort their transmissions in a contention.
Full duplex wireless communication also motivates researchers to migrate random backoff from the time domain to the frequency domain using the non-contiguous OFDM (NC-OFDM) technique. The method is to let a node occupy only one randomly selected subcarrier for preamble transmission. Contending nodes learn each other's random number and back off accordingly. In practice, the collision probability is high because only a few subcarriers are available for contention resolution and there is no complementary mechanism like binary exponential backoff (BEB) that can be used to reduce the probability that two nodes will choose the same subcarrier.
This section provides background information related to the present disclosure which is not necessarily prior art.